The M42 Fat Man was developed at Fort Strong near Boston, Massachusetts. Development began in February 2076, with the goal being the development of a man-portable tactical nuclear launcher easy enough for use by troops on the ground, to destroy reinforced and entrenched positions.[1][2] Problems manifested instantly, as warheads were simply too heavy to clear the blast radius fast enough to provide safety for the operator. Even stripping them to the minimal possible size failed to produce an acceptable effect. Instead, the development team decided to increase the power of the launcher unit, even considering using railgun technology to accelerate the warhead. The notion was largely academic, as it would require for the overburdened soldier to carry an additional power pack the size of a suitcase and turn the Fat Man into a crew-served weapon.[3] Concurrent development ran on the MIRV version of the launcher. Live-fire testing claimed lives, in fact, with two soldiers killed during a test. No remains were recovered and the commanding officer in charge of the project, General Brock, covered up the incident, ordering his subordinates to send sand packed in an urn to their relatives.[4]

The project hit a breakthrough in December 2076,[5] when the project lead discovered a solution for the launcher problems. Six hours, fourteen cups, and one concerned night shift lab crew later, the future M42 launcher was conceptualized: Based on the spigot mortar mechanism, the launcher would only have to accommodate a firing through and system, relying on mechanical launching, rather than chemical or electrical means. Much like the 130 year old PIAT launcher, the Fat Man warhead would be propelled by a sub-charge embedded inside the warhead, "catapulting" the warhead through the air to the target - giving plenty of range with the added bonus of not producing a smoke trail that could reveal the firing team to the enemy.[6]

By September 2077,[7] after one hundred test firings the weapon were complete and ready to be shipped to units overseas. A large number of units was manufactured and shipped together with the appropriate warheads to select locations throughout the United States.[8]

The standard M42 "Fat Man" Launcher is a man portable nuclear payload, anti-materiel weapon platform. It was designed for mid-ranged combat in urban centers where cover was high, but chances of survival were low.

It is heavy, especially in hardcore mode when combined with its mini nuke ammunition, but very powerful. In fact, it is so powerful that in most cases, the use of the Fat Man is complete overkill.

The Fat Man requires some getting used to, as the heavy mini-nuke projectiles will arc down; thus, if shot straight ahead, the mini nuke will land several meters from were it was shot, severely damaging and/or killing the player character. Otherwise, if aimed correctly, the Fat Man's trajectory has an effective range of roughly 150 meters. The weapon can be used outside of V.A.T.S. for indirect fire over a considerable distance.

Following detonation of the mini nuke, the immediate area will be moderately irradiated, reaching peaks of about 15 rads/second.

Quarry Junction - Lying near the edge of a radioactive pool to the west of the quarry.

Nopah cave - On the corpse of a super mutant master. Upon returning to the cave several days later, this super mutant may be found alive with another Fat Man and only a small amount of health. He continues to respawn every few days thereafter.

A Fat Man and 3 mini nukes can be pickpocketed or looted from a Boomer while they are sleeping in one of 3 tents near crops to the southeast of the mess hall & munitions storage. If killed, this Boomer respawns every 3 days, providing a steady source of mini nukes.

A Fat Man can be found on the body of a super mutant master near a cazador nest directly south of Jacobstown. This location is marked during the quest Bleed Me Dry. Facing away from the Jacobstown gate and climbing up and around the hills to the right will lead to the path. The area is also accessible by going west from Red Rock Canyon, over some very steep trails.

The names of Fat Man and Little Boy are references to the code names of the atomic bombs dropped by the United States on Japan during World War II: Little Boy was dropped on Hiroshima while Fat Man was dropped on Nagasaki. For this reason, the weapon's name has been renamed as the Nuka Launcher in the Japanese translation of both Fallout 3 and Fallout: New Vegas due to its relation to Japanese history. However, the Little Boy modification still retains its name.

The bell heard after reloading is actually the lunch bell at Bethesda.[9]

The designation M42 is shared with a couple other pieces of military equipment, such as the M42 Duster SPAA gun and the M42 submachine gun.

The M42 Fat Man is similar to the real-life M-28/M-29 Davy Crockett miniature nuclear launch device.

There is a glitch where if you kill an enemy that had their weapon equipped, then equip your Fat Man and go pick up the dead enemy's gun (from the ground, not their inventory). The ammunition from their dead body will transfer into mini nukes and be put into your inventory. Example: Kill a fiend shooting at you with a laser rifle, then on their dead body you will see their hands still clutching their weapon. Shoot the arm that is holding the weapon and it will pop onto the ground. Proceed to equip the desired gun needing your preferred ammunition, then pick up the laser rifle. How ever many microfusion cells were on the fiend's body will be transferred into the amount of mini nukes/alien powers cells/etc... put into your inventory. This can also be done with the alien blaster and almost if not all other ranged weapons. (This glitch works the best with a laser RCW so that you can get up to 60 mini nukes at a time. Tested Xbox 360, works if done correctly) Note: In order for this glitch to work you must have the patch deleted (pre-patch).

Sometimes when a second mini nuke is fired to a distant location, an explosion will not be seen.

If you cripple your legs and go into third person, it will look like your character is holding the Fat Man, but there will be nothing there. Instead, it will be laying on the ground near you or floating in the air. If you fire it the nuke will go in whatever direction it is aiming. Confirmed on Xbox 360.

Direct hit damage from the projectile does not get applied. This is a bug that applies to all non-hitscan projectiles that cause explosions.

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